Somodevilla/Getty Images(WASHINGTON) -- The FBI has questioned and cleared some 300 people since Friday and still no hard evidence has emerged to corroborate early alarms of a potential Sept. 11 anniversary terror attack, U.S. officials told ABC News, leaving potentially deadly questions unanswered and security still on high alert.

Last week, intelligence emerged from what several officials called a single "credible" source that there was an ongoing plot to launch a vehicle-born bomb attack on New York City or Washington, D.C., prompting a federal bulletin to law enforcement, public announcements by top U.S. officials and a nationwide manhunt for three men. Since the alarm was first raised, the CIA, FBI and a number of federal and local agencies have been unable to find any evidence to back up the original information.

And though none of the men have been found, the 10th anniversary of the Sept. 11, 2001 terror attacks passed relatively without incident on Sunday. Federal and local law enforcement officials said that at least for a while, they won't be backing off the heavy, high-profile security that surrounded the anniversary.

According to former White House counter-terrorism advisor and ABC News consultant Richard Clarke, the business of pulling back on the massive security effort without a resolution to the bomb plot could be tricky business.

"They have to unwind the heavy security very slowly and in gradual steps -- make it appear that some of the heavy security has gone away while actually keeping a lot of surveillance the public won't see," Clarke said.

In New York, police officials said they would maintain the tough security at least through Monday morning.

"The threat for me is fundamentally the same. It hasn't changed. We don't have really additional information to add," NYPD Commissioner Raymond Kelly told ABC News' New York affiliate WABC-TV on Sunday, referring to the alleged bomb plot. "But there's no reason to lessen our alert status."

The FBI will continue its expanded security effort through the day and perhaps as long as it takes to investigate the alleged bomb plot, law enforcement officials told ABC News. The Department of Homeland Security is following suit, maintaining additional security at major transportation hubs and federal buildings.

In addition to the FBI interviews, a public alert about the potential plot also drew hundreds of citizen reports about suspicious packages and individuals.

In two separate instances on the 9/11 anniversary, fighter jets were scrambled to escort passenger planes after passengers on the flights allegedly acted "suspiciously." But in both cases, the suspicious activity turned out to be non-terror-related -- one case of frequent bathroom trips, another of a couple "making out" in the lavatory, federal officials said Sunday.